A high-sensitivity polarimeter is demonstrated for application to gas-phase chirality measurement. This device is based on the physics of the eigenstates of a Fabry-Perot cavity, permitting improvement in the sensitivity with respect to the usual polarimeters. Typical measurements of rotations of 50 (±1) × 10(-)(5)° induced by the optical activity of (R)-(+)-limonene and (S)-(-)-limonene in the vapor phase are shown. A noise level corresponding to a rotation of 10(-)(6)° is experimentally demonstrated. Application to the polarimetric monitoring of an enantiomer mixing racemization of limonene in the gas phase is also presented.